Contemporary robotic work tools are becoming more and more advanced and are able to perform more and more advanced tasks such as executing advanced operation patterns. In the example of lawnmower robots the advanced working pattern may be a complicated mowing pattern based on the layout of a garden including bushes, garden islands and other structures. To successfully navigate such complicated areas some contemporary robotic work tools employ satellite navigation.
A satellite navigation or sat nav system is a system of satellites that provide autonomous geo-spatial positioning with global coverage. It allows small electronic receivers to determine their location (longitude, latitude, and altitude) to within a few meters, or even centimeters, using signals transmitted along a line-of-sight by radio from satellites. Receivers calculate the precise time as well as position and carrier phase, which can be used as a reference for scientific experiments. A satellite navigation system with global coverage may be termed a global navigation satellite system or GNSS (Global Navigation Satellite System).
The use of GNSS systems requires good reception of satellite signals to work reliably. The satellite signals may sometimes be blocked by buildings, roofs, awnings, foliage or trees. To improve the accuracy of GNSS systems a reference receiver, or beacon, within a short distance from the target receiver can be used. This is called differential GNSS. There are several DGNSS techniques, such as the classical DGNSS (or DGPS), the Real Time Kinematics (RTK) and the Wide Area RTK (WARTK).
However, the signal from a beacon may also be blocked by for example a house if the garden or other work area extends around the building.
It should be noted that similar problems exist also for other position determining devices, such as using optical beacons where the line of sight may be blocked in certain areas.
If the robotic work tool is unable to correctly receive the signals from the position determining system, the robotic work tool will be challenged to correctly navigate the work area and a satisfactory coverage of the work area may not be achieved by the robotic work tool.
A robotic work tool may be configured to use other navigation tools such as dead reckoning or deduced reckoning navigation systems when a satellite signal can not be received correctly.
Such navigation systems are well known and need no detailed description. Basically the robotic work tool determines its current position by determining a direction, for example through using a compass, and determining a distance travelled, for example by counting wheel turns.
However, such navigation is prone to mistakes due to environmental factors, such as slippery surfaces, causing the wheels to spin, magnetic influence, that may affect the compass, and other factors.
Especially the magnetic influence can affect compasses to a great degree and may also vary significantly over a work area. The magnetic influence may both be caused by external factors (such as the robotic work tool passing a magnetic object) or internal factors (such as battery placement).
Even though the problems above have been discussed for lawnmower robotic work tools, the same or similar problems exist also for other robotic work tools.
There is thus a need for a manner of enabling reliable operation of a robotic work tool even in situations where the robotic work tool may not be able to receive reliable and accurate positioning signals in all areas of a work area and where deduced reckoning may not be sufficiently reliant.